In the ink jet recording method, pulse signals are applied to a heater disposed in an ink-filled nozzle to heat the heater, to boil ink, and to cause the boiled ink to increase the vapor pressure to jet ink. To use this method on an image forming device, a plurality of nozzles are arranged to form one recording head, and a plurality of recording heads (for example, for jetting ink in cyan, magenta, yellow, black, and so on) are combined, to form a full-color image.
Conventionally, when forming an image using a plurality of recording heads by the ink jet recording method, the problem is that a horizontal deviation between any two recording heads, as shown in FIG. 14(a), is sometimes introduced when the recording heads are mounted on a carriage at a factory at shipment time or when a service engineer or a user replaces one or more recording heads. (In the example shown in the figure, the recording head of cyan (C) is deviated from the correct position by W). This deviation sometimes generates vertical stripes at print time and results in an unevenly printed image. Similarly, a vertical deviation between any two recording heads introduced when the heads are mounted, as shown in FIG. 14(b), sometimes generates horizontal stripes and results in an unevenly printed image.
In addition, on a device that uses a linear scale for establishing ink jet synchronization to jet ink at correct positions in the main scanning direction of the recording head, a jet position deviation (W2+W3) may occur during forward and backward printing depending upon the movement speed of the carriage, sometimes resulting in an uneven image printing. This deviation is caused by a delay generated before the ink is jetted from the time of passing the slit position as shown in FIG. 14(c).
Therefore, when a color registration error (hereinafter called a registration deviation) occurs through the recording head replacement or for some other reasons, the individual recording heads must be registered (i.e. registration adjustment). A registration deviation amount must be detected before making the registration adjustment. There are two methods of detecting registration deviation amounts: one is to print a particular test pattern, designed to make a registration deviation readily detectable, on paper so that human beings can check the print result to manually detect a registration amount, and the other is to cause a sensor to read a test pattern to detect a registration deviation.
The technology for reading the test pattern via a sensor is disclosed in Japanese Patent Laid-Open Publication No. Hei 7-323582. As shown in FIG. 15, the base recording head, one of a plurality of recording heads, and each of the other recording heads print a pattern made up of two parallel bars (pattern elements) to allow the sensor to read the same position of the parallel bars twice to detect the recording head deviation amount. That is, in the first scan, the sensor senses the width of each pattern element to calculate the center dot position thereof. Then, in the second scan, the sensor senses the width W1 between the pattern elements of the base head, based on the center dot positions of the pattern elements. Repeating the above-described operation for the pattern element of the base head and those of the other heads to calculate the widths (distances) W2, . . . , between the pattern elements of the base head and those of the other heads. Then, the head deviation amount ΔW is calculated based on the difference of those widths.
To do so, a comparator 1502 converts the analog signal, which is output from a sensor 1501, into a binary (bi-level) signal as shown in FIG. 16. In the first scan, this binary signal is sampled in a predetermined timing in accordance with a timer 1503. Each time a pattern element is read, a CPU 1505 references the value of the timer 1503 to read the pattern width data of each of two pattern elements. After the scan is terminated, the distance from the edge of the pattern element to the center dot is calculated from the scan speed and the sampling frequency, based on the width data of each of two pattern elements. After that, setting the center value of each pattern element in the timer 1503 immediately before the pattern is read in the second scan causes the timer 1503 to output a carry signal at the time the carriage reaches the center position of the pattern element. By operating a timer 1504 using this carry signal, the distance between the center dot position of a pattern element and that of another pattern element is calculated. This is done for the pattern elements of the base head and for the pattern elements of the base head and other heads to calculate the head deviation amount ΔW.
However, in this case, the signal is sampled in a predetermined timing. Therefore, the carriage speed varies during carriage scanning, from scan to scan, or from device to device due to various mechanical factors such as the tension of a drive belt connecting the carriage and the motor. This variation is accumulated in the sampling results, sometimes affecting the precision of registration adjustment. In addition, detecting each pattern-to-pattern width W1, W2, . . . requires the carriage to scan twice, thus requiring a long detection time and, at the same time, doubling the accumulation variation.
This applies also to the paper conveyance direction. Variations in the paper conveyance roller diameter, eccentricity, and gears connecting the motor to the roller generate accumulation variations in the accumulated sampling results.
In view of the foregoing, it is an object of the present invention to provide an image forming device capable of precisely detecting a recording head deviation when the recording head has been replaced.
In addition, variations in the shape or the direction of nozzles, introduced when manufacturing the recording head, will cause ink droplets to be jetted, not in exactly correct positions in a straight row as shown in FIG. 23(a), but in positions shifted vertically and/or horizontally as shown in FIGS. 23(b)–23(d). In the method described above in which a test pattern is read by the sensor to detect a registration deviation amount, two parallel pattern elements are printed by the base head and each of other heads. Then, the sensor reads the width of each pattern element from the edges of the pattern as well as the distance between the centers of the pattern elements of the base head and each of the other heads. Therefore, variations in the edges of pattern elements are generated as described above, and those variations generate read errors.
Also, when mounting a recording head 101 on a carriage 106, mechanical variations in the recording head 101 and the carriage 106 may cause the recording head 101 to be inclined with respect to the main scanning direction as shown in FIG. 24. In addition, the position at which a sensor 110 is mounted on the carriage 106 may vary according to the device. The recording head 101, if inclined with respect to the carriage 106, causes the pattern elements to be inclined as shown in FIGS. 25(b) and 25(c) although those pattern elements should be vertical bars (FIG. 25(a)). On the other hand, if the sensor read positions in the longitudinal direction of the pattern element vary as indicated by A to D, detection errors up to the value d occur.
As described above, there is a possibility in the conventional registration detection method that the pattern detection result varies greatly according to the manufacturing variations in the recording head 101, how the recording head 101 is mounted on the carriage 106, and how the sensor 110 is mounted.
Therefore, it is another object of the present invention to provide an image forming device capable of detecting a test pattern more precisely in order to precisely detect a head deviation when a recording head has been replaced.